Obesity has become a major public health concern, particularly among the veteran population, leading to increased morbidity and mortality from diabetes, cardiovascular disease, malignancies, and other complications. To date, there is no clearly effective and safe medication for long-term obesity treatment. Furthermore, dietary and behavioral modification often produces suboptimal weight loss among obese individuals, who predictably start returning to baseline body weight six months after initiation of reduction in body weight. One emerging therapy is the co-administration of leptin and amylin to obese subjects, a treatment which is currently being tested in combination with Amylin (Symlin(R)) Phase III clinical trials. However, obesity is considered a leptin resistant state. While administration of leptin at replacement doses to patients with leptin deficiency and congenital lipoatrophy (i.e., 'leptin sensitive' individuals) results in dramatic improvements in metabolic parameters (including insulin resistance, and/or hyperlipidemia) as well as in neuroendocrine and immune function, the exact intracellular signaling pathways mediating leptin's action in humans with these conditions remain unknown. Furthermore, whether leptin signaling defects in obese, leptin resistant subjects underlies leptin resistance has not yet been studied. A better understanding of these pathways could not only facilitate the identification of mechanisms that underlie leptin's action at the cellular level, but may also elucidate mechanisms of leptin action in low-leptin states and leptin resistance in hyperleptinemic states (e.g., obesity). To address these questions, we propose to conduct interventional studies in humans to: 1) determine whether leptin administration in vitro, ex vivo, and in vivo can induce the activation of intracellular signaling pathways (AMP kinase and other pathways) in adipocytes, muscle, and peripheral blood mononuclear cells, and 2) investigate whether dysregulation of the AMP kinase or other pathways downstream of the leptin receptor could be a mechanism for leptin resistance in obesity and/or diabetes in humans. Such studies could also provide targets for therapeutic interventions in leptin resistant states such as obesity not only by elucidating pathways of leptin action, but also by investigating potential interactions of leptin with the immune and neuroendocrine systems that may alter the signaling effects of leptin in vivo. This information will increase our understanding of leptin biology and more specifically the signaling pathways underlying leptin resistance in obesity, may prove to be important for improved clinical trial design, and has high therapeutic relevance as leptin is explored further as a therapy for either leptin deficiency or leptin resistant states. Moreover, elucidation of the intracellular signaling pathways underlying leptin resistance may eventually lead to the discovery of leptin sensitizers. This proposal can be materialized by the expertise of the Mantzoros lab in leptin biology, a leading research group that has recently completed several pivotal studies in human leptin physiology. The approach proposed herein is expected to result in novel mechanistic insights with the potential for translation into improved patient care and treatment for obesity associated disease states, problems of public health in the veteran population.